Masing Behavior and Microstructural Change of Quenched and Tempered High-Strength Steel Under Low Cycle Fatigue

doi:10.1007/s40195-019-00893-4

Abstract

Abstract:

Low cycle fatigue behavior of a quenched and tempered high-strength steel (Q960E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the dislocation structural evolution and fracture mechanism were examined and studied by transmission electron microscopy and scanning electron microscopy (SEM). The results showed that this Q960E steel showed cyclic softening at different strain amplitudes, and the softening tendency was more apparent at strain amplitude of ± (0.6-1.2)% than that at ± 0.5%. The reduction in dislocation density with increasing strain amplitude is responsible for the softening tendency of cyclic stress with the strain amplitude. The material illustrates near-Masing behavior at strain amplitude ranging from ± 0.6% to ± 1.2%. The near-Masing behavior of Q960E high-strength steel can be the result of stability of martensite lath at different strain amplitudes. Partial transformation from martensite laths to dislocation cells is responsible for the derivation from ideal Masing behavior. In the SEM examination of fracture surfaces, transgranular cracks initiate on the sample surface. Striations can be found during the crack propagation stage.

Key words: High-strength steel, Low cycle fatigue, Near-Masing behavior, Martensite lath

Feng-Mei Bai, Hong-Wei Zhou, Xiang-Hua Liu, Meng Song, Ya-Xin Sun, Hai-Long Yi, Zhen-Yi Huang. Masing Behavior and Microstructural Change of Quenched and Tempered High-Strength Steel Under Low Cycle Fatigue[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(11): 1346-1354.

Figures/Tables 14

Table 1 Chemical composition of Q960E steel (wt%)

C	Mn	Si	N	P	S	O	Cr+Mo	Nb+Ti+V Fe
0.18	1.5	0.5	≤ 0.004	≤ 0.008	≤ 0.002	≤ 0.0015	1.0	0.10 Bal.

Fig. 1 Microstructure of 900 MPa grade high-strength steel, a OM image, b TEM image

Table 2 Monotonic tensile properties and parameters of Q960E steel

0.2% yield strength, σ_0.2 (MPa)	Ultimate tensile strength, σ_UTS (MPa)	Total elongation, δ	Tensile strength coefficient, K (MPa)	Tensile strain-hardening exponent, n
975	1105	15%	660	0.16

Fig. 2 Specimen geometry used in low cycle fatigue tests

Fig. 3 Cyclic stress response curves of Q960E steel

Fig. 4 Comparison between tensile stress-strain curve and cyclic stress-strain curve from Half-life hysteresis loops at a range of total strain amplitude between?±?0.5% and?±?1.2%

Fig. 5 a Cyclic stress-strain curve obtained by connecting the tips of half-life hysteresis loops; b construction of master curve obtained by enveloping the upper branches of hysteresis loops

Fig. 6 Martensite lath structures under strain amplitude of?±?0.5%

Fig. 8 Martensite lath under strain amplitude of?±?1.2%, b an enlarged image from the red dotted box in (a)

Fig. 9 Martensite lath and dislocation cell structures under strain amplitude of?±?0.8%

Fig. 10 Martensite lath and dislocation cell structures under strain amplitude of?±?1.2%

Fig. 11 Schematic representation of evolution of dislocation structures under LCF loading in high-strength steel. PAGB is an abbreviation of prior austenite grain boundary

Fig. 12 OM micrographs taken from the surface of the specimen tested under the strain amplitude of?±?0.8%

Fig. 13 Fracture features of Q960E steel at Δεt?=?±?0.8%, a crack initiation, b crack propagation, c striations, c an enlarged image from the red box in b, d dimple fracture

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